Prior Art
Devices such as photodiodes used to detect and receive electromagnetic signals are well known and widely used. Generally speaking, the photodiode is used in an environment which highly restricts the variations in background or environmental lighting conditions that exist. For example, IR detectors used in TV set remote control receivers are usually recessed in cavities and those used in fiber-optic communications may be totally enclosed.
In the most basic receiver circuit, the resistance of a reverse-biased photodiode changes with incident light resulting in a change of the current in the circuit and the voltage that is developed across a resistor. The result of changing voltage across the resistor is, of course, a change in the bias voltage across the diode with a resultant change in the junction capacitance and a change in the frequency response of the diode. The value of resistance is chosen to provide the best signal-to-noise (S/N) ratio for a given diode. For a highly restricted range of light inputs, the total variation in junction capacitance may not be sufficient to prevent near optimum values for frequency response, S/N, and sensitivity over that light level range. Alternative circuits can be used for the purpose of holding the reverse bias constant via op amps or similar circuits.
In applications where the environmental incident light level may vary from total darkness to unshielded daylight, the problems discussed above become more severe. First, the current range through the diode may extend to the limits of device operation. Second, maintaining a constant bias voltage becomes more difficult over the wider current range. Third, the tradeoffs regarding current and current noise and the effects of resistor size on the S/N ratio become more serious when using the diode over a very wide range.
The desired circuit should have a high output impedence for the photodiode in order to provide maximum sensitivity. A constant bias voltage over the operating range is also needed to provide a constant frequency response. Finally, the photodiode must be able to operate over the widest possible current range without degrading frequency response or sensitivity and without requiring output voltage swings that are difficult to accomplish in practice. None of the prior art circuits are suitable for operation from direct sunlight to total darkness.